FACEOFF! The Moon’s oddly different sides (Astronomy, August 2014)

Clues unearthed by NASA’s Lunar Reconnaissance Orbiter and GRAIL spacecraft have opened a new window on the Moon’s creation and early evolution.

by Jesse Emspak

The Moon is so near that even a small telescope can show detail just a few miles across. Intrepid astronauts have explored our satellite up close with their own eyes, while spacecraft with senses that stretch far beyond anything humans possess have mapped its entire surface. We know largely what it’s made of, how old it is, and even where it likely came from.

But the Moon shows Earth only one face. Tidally locked to our planet, it rotates with the same period as it revolves around Earth. It wasn’t until 1959, when blurry images from the Soviet Union’s Luna 3 probe came back, that humans first saw the farside. It was stranger than anyone had thought.

The lunar maria — vast basins filled with frozen lava — that dominate the Full Moon’s appearance are mostly absent on the opposite side. There, the terrain is densely cratered, and the crust is far thicker. One of the solar system’s largest impact craters, the South Pole-Aitken Basin, rules the landscape. If astronauts had landed on the farside, no one could blame them for thinking they had dropped in on the wrong world.

Recent missions to the Moon have revealed a lot about the lunar surface and interior and provided valuable information for tackling the enigma of the Moon’s dichotomy. Like all good science, however, the new data raise as many questions as they answer — and have sparked a heated debate among lunar geologists.

Two sides of a coin

On the Moon’s Earth-facing hemisphere, maria cover some 30 percent of the surface. They form the “Man in the Moon” feature familiar in Western culture and the “Moon rabbit” celebrated in eastern Asia and by the Aztecs. The maria look darker because they are made of iron-rich basalts. Meanwhile, most of the lighter-colored highlands consist of anorthosite, a rock made largely of the mineral feldspar formed by magma as it cools and hardens.

The largest mare is Oceanus Procellarum, which stretches some 1,600 miles (2,500 kilometers) from north to south and covers about 1.5 million square miles (4 million square km). It is rich in a material called KREEP — an acronym that stands for potassium (whose chemical symbol is K), rare earth elements, and phosphorus.

While the anorthosite found in the nearside highlands is ubiquitous on the farside, mare basalts are nearly absent, covering less than 2 percent of the surface. If the Moon happened to be facing the other way, humans likely would have noticed only two maria, Moscoviense and Ingenii, while two others, Orientale and Australe, would show up near the lunar limb, or edge. The farside terrain is rough and covered almost entirely by craters. The largest impact feature, the South Pole-Aitken Basin, spans roughly 1,600 miles (2,500km) and digs down 5 miles (8km).

After Apollo, NASA didn’t return to the Moon until the Clementine and Lunar Prospector missions of the 1990s. Prospector underscored the nearside and farside differences. It found much higher concentrations of heat-producing radioactive elements such as thorium on the Moon’s nearside.

The main problem posed by this lunar dichotomy is that any process operating on a planetary (or large moon) scale shouldn’t have a preferential side. Impacts, for example, should occur equally on both hemispheres. Earth covers only a tiny portion of the Moon’s sky, so it makes a poor shield. Likewise, all things being equal, volcanism should affect the Moon uniformly.

The differences make even less sense in light of what scientists know about how the maria formed. These dark and relatively smooth regions arose when magma pooled in low-lying areas over millions of years. Typically, the basaltic lava filled large impact basins that formed when asteroids and other rocky bodies crashed into the Moon. The impact deformed the local crust, and lava welled up through fissures created in the basin floor. The South Pole-Aitken Basin should be covered with basalt — but it isn’t.

The South Pole-Aitken Basin should be covered with basalt — but it isn’t.

A decade after Prospector, Japan’s Kaguya spacecraft found that, in some ways, the two sides aren’t so different. The probe showed that the anorthosite is nearly pure and virtually the same over the entire Moon. Pure anorthosite occurs when rock melts, which means that a global magma ocean likely covered the young Moon and stamped its signature across the whole object.

A fresh look

NASA’s 2012 Gravity Recovery and Interior Laboratory (GRAIL) and its ongoing Lunar Reconnaissance Orbiter have given astronomers their clearest picture yet of the Moon’s mass distribution and lunar topography. The twin GRAIL spacecraft confirmed that the farside crust is roughly twice as thick as that of the nearside. The nearside figure typically ranges between 12 and 19 miles (20 and 30km) while the farside reaches about 37 miles (60km).

Planetary scientists have developed two sets of hypotheses to explain this asymmetry. One side advocates impact events; the other sees the Moon’s interior at work. Both groups begin with the Moon’s violent origin.

Most current theories say the Moon was born when a protoplanet at least the size of Mars, called “Theia,” hit the still-forming Earth about 4.5 billion years ago. The impact tore off our planet’s outer layers and hurled a cloud of molten debris into orbit. This material rapidly settled into a ring, where it coalesced into our satellite. The young Moon cooled quickly, and soon it developed a thin crust covering a magma ocean.

As it cooled, heavy elements sank toward the center while lighter ones floated to the surface in a process called “differentiation.” The mantle began to crystallize, though radioactive elements there generated heat and maintained its plasticity. During the Moon’s first several hundred million years, large impacts created the great basins, which later filled with magma bubbling up from below. Scientists think today’s Moon has a small iron-rich inner core surrounded by a liquid outer core wrapped by a thick mantle and topped by its uneven crustal skin.

The impact hypothesis explains why the Moon’s overall composition closely matches that of Earth’s mantle and why the combination of Earth’s spin and the Moon’s orbital speed is so high. The high temperature of the young Moon in this theory also explains the Moon’s relatively large abundance of elements that have high melting points. But the impact hypothesis by itself gives no reason why the lunar farside is so different.

A second big impact

In an August 2011 paper in Nature, planetary scientists Erik Asphaug and Martin Jutzi of the University of California, Santa Cruz, propose that the farside crust owes its existence to a collision with a second, smaller moon. They suggest that when Theia smashed into the young Earth, the debris ultimately coalesced into two satellites. Tens of millions of years later, the smaller moon “gently” collided with the larger one and accreted onto the farside.

“When I see … bimodal asymmetry, I think [it] must be the result of stochastic [random] events, which is kind of a code word for ‘giant impacts,’” says Asphaug, now at Arizona State University in Tempe. “If you took all the excess crust on the farside and rolled it up into a ball, it would be about 1,000 kilometers [600 miles] in diameter. In fact, that is the simple starting condition for our hypothesis.”

Asphaug and Jutzi ran a number of computer simulations. In many of them, the impact of a Mars-sized object into the early Earth created two satellites, and the smaller one frequently accreted at one of the socalled Lagrangian points that reside 60° ahead of and behind the Moon in its orbit around Earth. In these situations, the second satellite remained in a stable orbit for tens of millions of years, long enough for the larger Moon to cool and form a crust while the smaller moon solidified faster.

If this scenario is true, tidal forces from Earth would have relentlessly pushed the shared orbit of the two satellites farther from our planet. The Sun’s gravity eventually would have upset their orbital dynamics, and the second moon would have drifted from the Lagrangian point and approached the primary one. It would have done so slowly — Asphaug and Jutzi estimate the relative velocity at “only” 6,000 mph (9,000 km/h). At that speed, the impacting object would not have formed a crater. Instead, it would have acted more like a landslide, spreading out to cover one side of the Moon.

The computer simulations show that the colliding body would have needed only about .0 of the Moon’s mass to account for the farside’s excess crust. Still, the force of the impact would have been substantial. The shock wave from it would have pushed some material in the still-soft magma beneath the crust to the Moon’s opposite side. That would have boosted the concentration of radioactive elements on the nearside and account for the dominance of KREEP there.

“GRAIL found that the upper crust of the Moon is porous megaregolith [impactgenerated rubble], and one way for accounting for this high degree of … fragmentation is having it deposited as a ‘cosmic landslide’ when the second moon, a solidified object, crashes onto the [by this time] solidified crust of the Moon,” says Asphaug. “So I think the GRAIL gravity data [are] consistent with our hypothesis.”

As attractive as Asphaug and Jutzi’s idea is, however, it hasn’t convinced every lunar researcher. And one reason is the same GRAIL data that Asphaug says support it.

Hot and crusty

Maria Zuber of the Massachusetts Institute of Technology in Cambridge was the mission’s principal investigator. Her team’s analysis of the GRAIL observations points to a different explanation: The radioactive elements in the early Moon were not evenly distributed, and they caused the crust on one side to grow thinner.

The twin GRAIL spacecraft measured the Moon’s gravitational field with unprecedented accuracy. The method they used is quite simple. Any object in orbit around another moves at a certain velocity that depends on its altitude. If you know how high a satellite is orbiting and the mass of the object it’s going around, you know how fast it should be moving. But if one part of the Moon is more massive than it should be, the probe will move faster. By precisely measuring the relative velocity of GRAIL’s two satellites, scientists could deduce how much the gravitational pull varied across the lunar surface. This, in turn, allowed them to build a high-resolution map of the Moon’s gravity field that shows areas with more or less mass.

A projectile of equal size would make a crater twice as big on the nearside.

In a November 2013 Science paper, Zuber, Katarina Miljkoviæ of the Paris Institute of Earth Physics, and their colleagues looked at the sizes and locations of large lunar impact basins. They studied regions at least 125 miles (200km) across where the crust looked thinner than normal. “Every basin has a gravity anomaly … after you subtract the surface topography,” says Zuber. “What that anomaly indicates is thinning of the crust because it is less dense than the mantle. … That allows us to go in and measure how many basins there are.”

By counting the big ones, the researchers found that there are many more on the nearside. This is precisely what one would expect if the nearside’s crust were hotter and thinner than the farside’s, even if the same number of impacts rained down on both sides. In fact, a projectile of equal size would make a crater twice as big on the nearside.

Miljkoviæ, Zuber, and their team proposed that when asteroid-sized objects hit the thinner nearside crust, the deeper and larger craters they gouged out made it easier for magma to seep up and fill the basins, creating the maria. “What impacts did do is thin the crust in the center of basins, and heated and uplifted the underlying mantle,” says Miljkoviæ. “That may have later [on a geologic timescale] served as good grounds for volcanism; hence the maria later formed such that they coincided with the inner depression of the basins.”

But why would one side have been hotter in the first place? In their model, the Moon wasn’t a uniform sphere when it formed. The radioactive elements that produced heat were concentrated on one side of the Moon, and they produced more volcanism in those areas. On top of that, the Moon’s mantle may have produced a local plume of hot material. “Think about plumes on Earth under the East African Rift Valley or Hawaii, or [the giant] Tharsis bulge on Mars,” says Zuber. It’s not outlandish to suppose that something similar might have occurred in the Moon’s early history.

The fact that we see the maria-bearing face is by pure chance. Because the Moon’s mass was distributed unevenly from this young age, our satellite behaves more like a dumbbell than a sphere. Earth’s tidal forces eventually oriented it so the longer axis points toward our planet, and we just happened to get the “Man in the Moon” side.

Asphaug remains dubious. “I expect it would be hard to squeeze out so much excess volcanism — nearly [a] 1,000-kilometerdiameter [600 miles] planet’s worth — onto only the nearside of the Moon,” he says.

Zuber, for her part, says that Asphaug’s model would mean that mantle material should be present on the Moon’s surface. She notes that the Kaguya mission mapped the distribution of olivine, the mineral that most scientists think makes up the bulk of Earth’s mantle. “If the impactor did have the same composition as the mantle, we ought to be able to see olivine in farside highlands,” she says — and there doesn’t seem to be enough.

Impacts may yet have their day, however. A third model looks to the South Pole-Aitken Basin for answers.

The farside’s giant basin

Peter Schultz, a geologist at Brown University in Providence, Rhode Island, thinks that the South Pole-Aitken Basin — or rather the object that made it — did enough damage to the Moon to create the different faces we see today. In his scenario, published in the Geological Society of America Special Papers in 2011, the early Moon used to be much more uniform. In fact, it looked a lot like the current farside.

Schultz envisions a giant asteroid hurtling toward the Moon about 4.3 billion years ago and crashing into the south polar region at an angle. By this time, he says, the Moon would have cooled from its initial molten state, but the mantle’s radioactive elements wouldn’t have been concentrated enough yet to generate significant heat.

The blast would have sent shock waves through the entire Moon — all the way to the other side, where they would have deformed the crust. Crucially, the effects wouldn’t have occurred at the so-called antipode opposite the basin. “The [South Pole-Aitken Basin] exhibits features consistent with an oblique trajectory from the northwest,” he says. “In addition, the offset antipode is centered on the well-known semicircular pattern of [features] across the western side of the Moon facing us.”

Schultz and his co-author, David Crawford of Sandia National Laboratories in Albuquerque, New Mexico, tested the model at NASA’s Ames Vertical Gun Range at Moffett Field, California. They fired a tiny glass sphere at a larger acrylic one and observed the patterns of cracks. Schultz says that if the impact isn’t perfectly vertical, most of the damage from an impact is offset from the point directly opposite it.

All the cracking and shaking would have caused magma to seep toward the lunar surface. Although it wouldn’t have broken through, it would have come close enough to the surface that other impacts would have let it out, forming the maria.

To get the final word on any of these models likely will mean sending more spacecraft to our next-door neighbor. To confirm the Asphaug – Jutzi scenario of a second moon’s impact, for example, a sample return from the farside probably would be necessary. Schultz says he would want to put a series of seismometers on the Moon to get a better idea of its internal structure — and to see if the cracks beneath the crust follow the pattern his theory says it should.

The truth might well involve a combination of several models — reflecting the complicated history of the solar system. “We simulated the accretion of two moons to make the Moon; that does not mean that geology ended there,” says Asphaug. “Thermal … evolution in response to cooling, further differentiation, and Earth’s tides would begin where our modeling ends.”